The use of orthogonal frequencies for a wealth of communication and signal processing applications is well known to those skilled in the art. Orthogonal frequencies are often used in an M-ary frequency shift keying (FSK) system. There is a required relationship between the local, or basis set, of frequencies and their bandwidths that meets the orthogonality condition. If adjacent time chips have contiguous local stepped frequencies, then a stepped chirp response is obtained.
Other orthogonal frequency coding techniques by the same inventor, and assigned to the same assignee, include U.S. Pat. No. 7,642,898 which disclosed a novel spread spectrum coding that uses orthogonal frequency coding (OFC) for surface acoustic wave identification tags and sensors which enables unique sensor operation and identification in multi-sensor environments. The OFC technique provided a wide bandwidth spread spectrum signal with all the inherent advantages obtained from the time-bandwidth product increase over the data bandwidth.
U.S. Pat. No. 7,623,037 issued to the same inventor, and assigned to the same assignee, teaches weighted SAW gratings for coding identification tags and sensors for a multi-sensor environment. In an embodiment, the weighted reflectors are variable while in another embodiment the reflector gratings are apodized. The weighting technique allows the designer to decrease reflectively increases coding diversity.
U.S. Pat. No. 7,623,037 issued to the same inventor, and assigned to the same assignee, teaches UCF-560CIP teaches a method to mitigate code collisions in a wireless multi-tag system, each one of the OFC surface acoustic wave devices generating an orthogonal frequency coded signal for identification
The co-inventors previously published results showing that the cross correlation between differing orthogonal frequency coded frequency chips yield reduced correlation sidelobes compared to conventional Code Division Multiple Access (CDMA). The co-inventors have also discussed the use of both frequency division (FD) between tags and time division (TD) of chips in tags. Both of these approaches were considered since each provides greater diversity of coding in a multi-sensor system. Both approaches also provide some measure of orthogonality, based on the implementation technique, over and above that of just OFC coding.
The present invention solves the problem of extracting time delays injected into plural coded signals in a multi-device system that generates and transmits a coded signal in response to an interrogation signal. The analysis assumes all tags of equal signal strength received at the antenna, having a known single range. The present invention uses correlator time delay extraction with matched filters to remove the block time delay to recover each original coded signal.